1. Field of the Invention
The present invention relates to a zoom lens, and particularly to a projection zoom lens to be applied to a projection type display device.
Further, the present invention relates to a projection type display device equipped with such a projection zoom lens.
2. Description of the Related Art
Conventionally, projection type display devices that utilize light valves, such as liquid crystal display elements, DMD's (Digital Micromirror Device: registered trademark), or the like have been widespread. Further, in recent years, movie theaters and the like have been using such projection type display devices, which are designed to be capable of being applied to large screens and of producing high-definition images. Such projection type display devices to be utilized in movie theaters and the like as described above have been required to have long back focus and a favorable telecentricity.
This is because such projection type display devices adopt a three-plate system in which three light valves for the respective primary colors are provided. In the three-plate system, a light beam from the light source is divided into three primary colors by a color separation optical system, and the separated light of the three colors passes through the respective light valves and then is composed by a color composition optical system to be projected.
Further, such projection type display devices to be utilized in movie theaters and the like have been required to be equipped with high zoom-ratio zoom lenses as a projection lens. This aims at fitting the size of display images to a screen size in accordance with the projection distances and screen sizes, which differ depending on movie theaters, halls, and in accordance with aspect ratios (cinema scope, Vista size) of display images.
Further, there are many cases that such projection zoom lenses are required to maintain constant numerical apertures (hereinafter, referred to as “F number(s)” as well) over the entire zoom range. This is because operating in such a manner can prevent the brightness of display images from changing when the zoom ratios of this type of zoom lens are changed to match the sizes described above.
Further, accompanying the rapid pace of digitalization of cinema screens, projection type display devices have been rapidly miniaturized and reduced in cost. There is likely to be also demand for projection zoom lenses to be miniaturized and reduced in cost in addition to the demands for back focus, telecentricity, and high zoom ratio.
Patent Documents 1 through 5 (Japanese Unexamined Patent Publication No. 2006-184723, Japanese Unexamined Patent Publication No. 2005-106948, Japanese Unexamined Patent Publication No. 2008-046259, Japanese Unexamined Patent Publication No. 2009-128683, and Japanese Unexamined Patent Publication No. 2012-058607) disclose zoom lenses which are intended for use in projection type display devices. More specifically, Patent Document 1 discloses a projection zoom lens of a six-group configuration. In the six-group configuration, a first lens group having a negative refractive power disposed at the most-magnification side and a final lens group having a positive refractive power disposed at the most-reduction side are fixed while changing magnification; and an aperture stop is disposed within a fourth lens group.
Meanwhile, Patent Document 2 discloses a projection zoom lens in which lens groups positioned at the reduction side of the aperture stop are fixed while changing magnification so that a constant numerical aperture is maintained. Further, Patent Document 3 discloses a projection zoom lens in which a first lens group having a negative refractive power disposed at the most-magnification side and a final lens group having a positive refractive power disposed at the most-reduction side are fixed while changing magnification. The projection zoom lens of Patent Document 3 is further configured to maintain a constant numerical aperture over the entire zoom range by using a variable stop that varies the aperture diameter thereof accompanying changes in magnification.
Further, Patent Documents 4 and 5 disclose projection zoom lenses, in which a first lens group having a positive refractive power disposed at the most-magnification side and a final lens group having a positive refractive power disposed at the most-reduction side are fixed while changing magnification, and moving lens groups disposed between these fixed lens groups are caused to move while changing magnification. The projection zoom lenses of Patent Documents 4 and 5 having such a configuration maintain constant numerical apertures over the entire zoom range.